Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Bring water to a boil. Add cauliflower, cover, and steam until tender, 15 to 20 minutes. Drain and let cool enough to handle, 5 to 10 minutes.
Preheat the oven to 400 degrees F (200 degrees C). Lightly butter a casserole dish.
Transfer florets into the bowl of a food processor. Add broth and puree on high speed until smooth, but with some very small pieces remaining for just a bit of texture.
Melt butter in a saucepan over medium heat until a pinch of flour sprinkled into the oil will just begin to bubble. Whisk in flour to form a thick paste the consistency of cake frosting. Continue cooking, whisking constantly, until flour has turned golden and begins to smell like toast, 3 to 5 minutes.
Stir in pureed cauliflower, cream, sugar, salt, white pepper, garlic powder, and onion powder. Set over medium heat and cook, stirring often, until thick, 5 to 10 minutes. Transfer to the prepared dish and sprinkle with Parmesan cheese.
Bake in the preheated oven until golden and cheese is melted, 10 to 15 minutes.